1. Field of the Invention
The present invention relates to a solid-state image sensing apparatus for converting a light signal into an electric signal, and more particularly, to a solid-state image sensing apparatus where the dynamic range is expanded by the incorporation of a logarithmic conversion function. p 2. Description of the Prior Art
Solid-state image sensing apparatuses not only are smallsized, light-weight and consume less power but they also are free from image distortion and sticking and have a tolerance for environmental conditions such as vibration and magnetic fields. Since they can be manufactured by a process common or similar to that of large scale integrated circuits (LSIs), they have a high reliability and are suitable for mass production. Because of these advantages, solid-state image sensing apparatuses are widely used. For example, a linear image sensing apparatus is used in a facsimile apparatus and a two-dimensional image sensing apparatus is used in a video camera.
However, most solid-state image sensing apparatuses have defects that a precise control of the exposure amount is required since the dynamic range is small compared to that of the silver film and that even if the exposure amount is precisely controlled, a dark portion of an image is sensed as a black portion, or a bright portion is sensed as a white portion, since the dynamic range is small. As a solution to these problems, a solid-state image sensing apparatus has been proposed having a wide dynamic range and capable of sensing light of from high luminance to low luminance. The solid-state image sensing apparatus is provided with a photosensitive means capable of generating a photoelectric current which is in accordance with the intensity of incident light, a metal oxide semiconductor (MOS) transistor to which the photoelectric current is inputted and a bias means for biasing the MOS transistor so that the bias voltage thereof is equal to or below a threshold voltage and that the subthreshold current can flow therethrough. A characteristic region where the subthreshold current flows is hereinafter referred to as a subthreshold region. The logarithmic compression conversion of the photoelectric current is made by using the MOS transistor in a subthreshold region.
FIG. 1 shows an arrangement of a circuit corresponding to one pixel of a solid-state image sensing apparatus of Japanese Patent Application H1-334472 of the present applicant.
In this circuit, when V.sub.O =V.sub.OI at t=0, if the substrate bias effect is ignored, the following equation (1) is obtained: ##EQU1## where: q is an electron charge;
k is a Boltzman's constant; PA1 T is an absolute temperature; PA1 n is a constant determined by the configurations of MOS transistors 2a and 2b; and PA1 C is a capacitance of a capacitor 3. PA1 .sub.DO is a constant determined by the configuration of the first MOS transistor 2a.
The equation (1) indicates that the sum Of an accumulation value of a photoelectric current I.sub.P and a fixed value determined by V.sub.OI -V.sub.SS1 is logarithmically converted into a voltage V.sub.O. If V.sub.OI -V.sub.SS1 is sufficiently small, the equation (1) can be re-written as: ##EQU2## Thus, the logarithmic conversion of the sum is precisely made.
As described above, according to the prior art (Japanese Patent Application No. H1-334472), the accumulation value of the photoelectric current I.sub.P is logarithmically converted. That is, even if the intensity of light incident onto a photodiode 1 changes during the accumulation and the photoelectric current I.sub.P changes accordingly, the accumulation value of the photoelectric current I.sub.P is logarithmically converted (in a typical solid-state image sensing apparatus which has no logarithmic conversion function, a signal proportional to the accumulation value of the photoelectric current I.sub.P is obtained). With such a feature, a solid-state image sensing apparatus having a wide dynamic range and capable of sensing light from high luminance to low luminance can be realized.
In the prior art, however, when the intensity of light incident onto the photodiode rapidly changes, the output voltage V.sub.O cannot sufficiently follow the change of the light intensity. This results from the fact that a stray capacitance 5 exists at a node 4 of the MOS transistors 2a and 2b as shown by the dotted lines in FIG. 1. That is, in order to obtain the equation (1) or (2), it is necessary, as described in the specification of the prior art, that a gate voltage V.sub.G of the first and second MOS transistors 2a and 2b change in correspondence with the photoelectric current I.sub.P as indicated by the following equation (3): ##EQU3## where: V.sub.T is a threshold voltage of the first MOS transistor 2a; and
Under a stationary state, the gate voltage V.sub.G is determined by the equation (3) and no currents flow through the stray capacitance 5. However, when the photoelectric current I.sub.P changes, a current for the charging or discharging of the stray capacitance 5 flows through the stray capacitance 5. The current becomes 0 when the charging or discharging is completed, and consequently the gate voltage V.sub.G takes a value obtained by the equation (3). For this reason, the change of the gate voltage V.sub.G is delayed (delayed by the period of time required for the charging or discharging of the stray capacitance 5) compared with the change of the photoelectric current I.sub.P. Hence, when the photoelectric current I.sub.P changes, the logarithmic conversion based on the equation (1) or (2) is not precisely made.
The delay of the change of the voltage V.sub.G will be described in more detail. First, a case will be described where the photoelectric current I.sub.P decreases due to a decrease of the light intensity. In this case, the charge accumulated in the stray capacitance 5 is discharged, so that the potential of the voltage V.sub.G decreases. This discharging is made by means of a drain current of the first MOS transistor 2a. The drain current decreases as the gate voltage decreases. Therefore, the drain current decreases as the drain voltage, i.e. the gate voltage decreases with the proceeding of the discharging of the charge accumulated in the stray capacitance 5. Because of the drain current decrease, the effect of the discharging deteriorates, thereby increasing the discharging time. For this reason, the capability of the gate voltage V.sub.G to follow the change of the photoelectric current I.sub.P deteriorates.
In a case where the photoelectric current I.sub.P increases due to an increase of the light intensity, the potential of the gate voltage V.sub.G increases as charge is accumulated in the stray capacitance 5. The current required for the charging is supplied by the photodiode 1. However, since the current I.sub.P which flows through the photodiode 1 is divided into a current which flows through the stray capacitance 5 and a current which flows through the first MOS transistor 2a, the effect of the charging deteriorates as the current which flows through the first MOS transistor 2a increases. Since the gate of the first MOS transistor 2a is directly connected to the drain thereof, the gate voltage increases as the drain voltage increases, thereby deteriorating the effect of the charging. For this reason, the capability of the gate voltage V.sub.G to follow the change of the photoelectric current I.sub.P deteriorates.